Electrophotographic dry developing systems employed in copying machines, MFPs or other types of electrophotographic apparatuses are categorized into a system using a one-component developer containing only toner and a system using a two-component developer containing toner and carrier. In either of these developing systems, toner charged to a predetermined level is applied to a photoreceptor. An electrostatic latent image formed on the photoreceptor is rendered visual with the toner and is transferred to a sheet of paper. The image visualized by the toner is fixed on the paper to obtain a desired image.
A brief description about development with the two-component developer will be given. A predetermined amount of toner and a predetermined amount of carrier are accommodated in a developing apparatus. The developing apparatus is provided with a rotatable magnet roller with a plurality of south and north poles alternately arranged thereon in the circumferential direction and an agitation roller for agitating and mixing the toner and carrier in the developing apparatus. The carrier made of a magnetic powder is carried by the magnet roller. The magnetic force of the magnet roller forms a magnetic brush, which is also called straight-chain like bristles. Agitation produces triboelectric charges that bond a plurality of toner particles to the surfaces of the carrier particles. The magnetic brush abuts against the photoreceptor with rotation of the magnet roller to supply the toner to the surface of the photoreceptor. Development with the two-component developer is carried out as described above.
The recently dominating carrier includes carrier core particles that are the core, or the heart of the carrier particles, and coating resin that covers the outer surface of the carrier core particles. The carrier, which is a component of the two-component developer, is required to have various functions including: a function of triboelectrically charging the toner by agitation in an effective manner; a toner transferring ability to appropriately transfer and supply the toner to the photoreceptor; and an improved charge transfer rate at which residual charge on the carrier surface after toner has been transferred to a photoreceptor is leaked.
The carrier in the developing apparatus is carried by the magnetic force of the magnet roller. In such usage, as the retentivity of the carrier to the magnet roller decreases, so-called carrier scattering occurs, or more specifically, the carrier scatters toward the photoreceptor, resulting in adhesion of the carrier on paper where an image is formed.
Technologies to prevent the carrier scattering are disclosed in Japanese Unexamined Patent Application Publication Nos. 2002-296846 (PTL 1) and 2008-191322 (PTL 2).
In the carrier for electrophotographic developer according to PTL 1, the volume mean diameter of spherical magnetic carrier core particles is 25 to 45 μm, the mean pore size of the carrier particles is from 10 to 22 μm, the ratio of particles having a diameter of 22 μm or lower based on a volume size distribution measurement is less than 1%, the magnetization in a magnetic field of 1 kOe is 67 to 88 emu/g, and the difference in magnetization between scattered carrier particles and original carrier particles in a magnetic field of 1 kOe is 10 emu/g or lower. The carrier having such compositions can prevent image degradation caused by hardening of the bristles of the magnetic brush, as well as carrier scattering.
PTL 2 discloses carrier for two-component type electrophotographic developer invented to make the magnetic brush flexible to mitigate the adhesion of the carrier to paper and improve the tone reproducibility of images. To achieve such carrier, the volume mean diameter of the carrier particles is set to 15 μm to 40 μm, the ratio of carrier particles having a diameter less than 22 μm is set to 1.0% or more, the fluidity of the carrier particles is set to 30 sec/50 g to 40 sec/50 g, and the apparent density of the carrier particles is set to 2.20 g/cm3 to 2.50 g/cm3.